1. Field of the Invention
The present invention relates to a method for calibrating a lithographic apparatus. The present invention further relates to methods of manufacturing devices using lithographic apparatus calibrated by such a method, and to data processing apparatuses and computer program products for implementing parts of such a method.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it and critical line width in a developed metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
In order to better control scanner functionality, a module has been recently devised which automatically drives the system towards a pre-defined baseline each day. This scanner stability module retrieves standard measurements taken from a stability monitor wafer using a metrology tool. The monitor wafer had been previously exposed using a special reticle containing special scatterometry marks (or marks suitable for whatever other metrology tool is to be used). Using the monitor wafer and that day's measurements (and possibly the historical measurement data from the previous days), the scanner stability module determines how far the system has drifted from its baseline, and then calculates wafer-level overlay and focus correction sets. The baseline can be defined either directly by the reference layer on the monitor wafers (in this case the scanner stability module will drive the system towards minimal overlay on the monitor wafers) or indirectly by a combination of the reference layer on the wafers and the target overlay fingerprint (in this case the scanner stability module will drive the system towards defined target overlay fingerprint on the stability monitor wafers). The lithography system then converts these correction sets into specific corrections for each exposure on subsequent production wafers.
While the scanner stability module using monitor wafers attends to stability of performance of a lithography tool, and may be performed daily, there remains the separate need to perform initial calibration of the tool when it is installed, and after any major interruption in operation. This calibration is done by conventional processes using specially manufactured reference wafers. After an interruption in operation, the calibration to reference wafers is performed again, after which the scanner stability module is reset by exposing and monitoring a stability monitor wafer. Given that any interruption of production using these expensive tools is very costly, users currently have to weigh up the benefits of the stability module against the cost of this extra calibration step. Another loss in productivity arises when the monitor wafer wears out and a new monitor wafer must be made with high accuracy. Further, there is the overhead and risk attached to the need for monitor wafers to be made and maintained with specific tools, in a facility where several tools may be operating side-by-side.